She added: "Since last year, the UK Government has provided funding to bolster the security and infrastructure of ports in Northern France - including a £2 million upgrade of detection technology, a £1 million extension of dog searching capacity and an additional £12 million to reinforce security at the juxtaposed border."

It is hoped the fencing in Calais and Coquelles will secure one mile of Eurotunnel's shuttle platform area where migrants attempt to board waiting and slow-moving trains destined for the UK.

Further improvements to halt migrants crossing illegally into Britain were taking place "in the coming weeks and days", the Prime Minister said.

He said: "We have done a lot in recent days to improve the situation but there's a lot more to do.

"So we have got more fencing, we've got more police officers, more sniffer dogs, more guards, better security, and we are making progress.

"But there's a lot more to do, including better security in the tunnel itself and myself and the team of ministers I have put in place, we will oversee these improvements and they will take place in the coming weeks and days."

His comments came as it was revealed that ministers had discussed plans to close the 31-mile Channel Tunnel if the crisis in Calais worsens.

The proposal has been discussed at the Government's emergency Cobra meetings in what has been described as the "nuclear option", the Daily Telegraph reported.

Sources said the option to close the Channel Tunnel - operated by Eurotunnel - exists but there were no immediate plans to do so.

A Government spokesman said the situation was under constant review.

The disclosure came as it emerged a suspected illegal immigrant from Sudan walked nearly the entire length of the Chunnel from Calais before being held by British officers.

Kent Police said Abdul Rahman Haroun, 40, has been charged with causing an obstruction to an engine or carriage using the railway under the Malicious Damage Act 1861.